A number of methods have been advanced to produce controlled porosity on micrometer and millimeter length scales. Principal among these methods is the use of colloidal templating in which micrometer sized beads of silica or polymers such as polystyrene are used as template for sol-gel mixtures, primarily silica. The sol-gel mixture may or may not include a structure directing agent to give porosity on the nanometer scale. After the removal of the polymer and nanostructure directing agent by dissolution or combustion, a three dimensional replica is produced. These replicas have regular, highly ordered micropore networks, are typically not mechanically very strong, and are difficult to produce in large bodies. The need for polymer or silica beads increases the cost of this synthesis and limits the range of structures available. The need for infiltration of the sol-gel precursor through the material also limits the applicability of this approach. The material formed by this approach has been used as a template for the formation of metal oxide and metals. Such materials have low surface areas.
Other methods for producing porous metal materials known as metal foams include bubbling gas through molten metal, generating gas during an exothermic reaction that melts the metal or that reduces and melts the metal, and generating gas during electrodeposition of the metal. Such processes generally produce large pore size distributions, are not compatible with the simultaneous formation of nanopores, are typically not very mechanically robust, and often have significant amounts of organic impurities. Other methods of making metal foams involve electrodeposition or metal condensation onto carbon skeletons. These approaches typically are not capable of generating small pore diameters and large bodies.
Other methods have been developed for producing porous metal oxide particles. Yue et al. (“Synthesis of Porous Single Crystals of Metal Oxides via a Solid-Liquid Route”, Chem. Mater. 19:2359, 2007, and “Mesoporous metal oxides templated by FDU-12 using a new convenient method”, Studies Surf Sci. Catal, 170:1755, 2007), disclose that metal nitrates can be introduced into mesoporous silica particles by grinding the metal nitrate and mesoporous silica particles in a crucible and then heating the material at a rate of 1° C./min to 500° C. where the temperature is maintained for 5 hours. Yue et al. teach that the metal nitrate melted and entered the pores of the silica. They disclose images of small (<150 nm) sized particles. Yue et al.'s disclosure demonstrates a limitation of that method for preparing larger bodies. For example, the grinding step reduces the size of the mesoporous silica particles limiting the size of replica particle that can be produced. The grinding step would result in the destruction of a larger body. The heating ramp used also constitutes a limit on the size of replica that can be achieved as it provides insufficient time for transport of materials within the mesopores. As such, these methods have not been applied to larger porous bodies.
Moreover, while the decomposition of metal nitrates has been much studied, the results have been contradictory. As the temperature of a nitrate melt is raised towards the decomposition temperature of the nitrate, water of hydration can be lost producing compounds that may either be a liquid or a solid and so may or may not be able to move within a mesopore. The loss of water is determined by the pressure of any ambient atmosphere and by the rate of transport of the water vapor through the mesopores. This can be illustrated by considering the nickel nitrate system which is among the most studied metal nitrates. Heating nickel nitrate is reported to go through a series of dehydration steps (Brockner et al., “Thermal decomposition of nickel nitrate hexahydrate, Ni(NO3)2.6H2O, in comparison to Co(NO3)2.6H2O and Ca(NO3)2.4H2O”, Thermochim. Acta 456:64, 2007; Llewellyn et al., “Preparation of reactive nickel oxide by the controlled thermolysis of hexahydrated nickel nitrate”, Solid State Ionics 101:1293, 1997; Mansour, “Spectroscopic and microscopic investigations of the thermal decomposition of nickel oxysalts. Part 2. Nickel nitrate hexahydrate”, Thermochim. Acta 228:173, 1993; Paulik et al., “Investigation of the Phase Diagram for the System Ni(NO3)2—H2O and Examination of the Decomposition of Ni(NO3)2.6H2O”, Thermochim. Acta 121:137, 1987; and Estelle et al., “Comparative study of the morphology and surface properties of nickel oxide prepared from different precursors”, Solid State Ionics, 156:233, 2003). Various mechanisms of decomposition of the nickel nitrate to nickel oxides are reported. (Brockner et al., Thermochim. Acta 456:64, 2007; Llewellyn et al., Solid State Ionics 101:1293, 1997; Sietsma et al., “Ordered Mesoporous Silica to Study the Preparation of Ni/SiO2 ex Nitrate Catalysts: Impregnation, Drying, and Thermal Treatments”, Chem. Mater. 20:2921, 2008; and Sietsma et al., “How nitric oxide affects the decomposition of supported nickel nitrate to arrive at highly dispersed catalysts”, J. Catal. 260:227, 2008). Further, the products and the mechanism are both reported to depend upon the atmosphere under which the heating to decomposition takes place. Sun et al. (“Container Effect in Nanocasting Synthesis of Mesoporous Metal Oxides”, J. Am. Chem. Soc. 133:14542, 2011) teach that even the shape of the container in which an amount of porous silica particles containing a metal nitrate melt is heated can change the structure of the metal oxide formed within the porous silica particles. Seitsma et al. (Chem. Mater. 20:2921, 2008 and J. Catal. 260:227, 2008) teach that carrying out heating of nickel nitrate under different atmospheres can affect the extent of migration of the nickel oxide product from the mesopores in silica particles onto the exterior of the particle. The affect of such variability in the decomposition and migration of metal nitrates has meant that these techniques have only been applied to particles, where the effects, though present, are more manageable and have less effect on the small scales seen with particle products. Such methods have not been applied for the preparation of larger porous materials.
Accordingly, there are no known methods for generating porous metal and/or metal oxide materials in which the spatial distribution of different metals and metal oxides within one material can be controlled. The methods disclosed herein can produce metal and/or metal oxide porous materials (e.g., bodies) having precisely controlled microstructure and nanostructure that includes control over the spatial distribution of a number of metal and metal oxides within the same material. The disclosed porous materials can be used in a variety of applications and can also incorporate carbon or silica present in some templates.